1. Field of the Invention
The present invention refers to roofing granules and roofing products.
2. Brief Description of the Prior Art
Asphalt shingles are conventionally used in the United States and Canada as roofing and siding materials.
Asphalt shingles can be classified into two types of shingles according to the nature of the reinforcement. “Organic” shingles contain cellulose or wood fiber as a thick fiber felt. “Glass fiber” shingles contain a nonwoven mat of glass fibers held together by a binder that is insoluble in water.
In the manufacture of organic shingles, a continuous web of organic fiber felt is fed from a supply roll to an accumulating device made up of several rollers, and then immersed in a first liquid asphalt bath having a temperature of about 250° C. After leaving the first liquid asphalt bath, the felt passes through a second accumulating device so that the felt can absorb excess asphalt and cool slightly. The so-impregnated felt is then coated with molten asphalt on each of its two faces, which ultimately become respectively the upper and lower faces of the web. Roofing granules are distributed on the upper face, and an anti-adhesive agent, for example, talc, is applied to the lower face. The resulting web passes between the rollers of a cold calendar so as to partially embed the roofing granules in the hot asphalt layer on the upper face of the web, and the subsequently cooled product is collected in the form of rolls or of sheets cut to the desired dimensions.
Except for the first stage of impregnation, which is omitted, the manufacture of the glass fiber shingles is carried out in the same way.
In the shingle, the asphalt functions principally to make material impervious to water. It is also used to support the granules and to give strength to the material. The highly ductile character of the asphalt-impregnated felt makes it possible to obtain a flexible product. In general, the longevity of the shingle increases with the quantity of asphalt employed.
The roofing granules, in general formed from mineral materials, serve to provide the shingle with durability. They protect the asphalt from the effects of the solar radiation (in particular from the degradative effects of ultraviolet rays) and of the environment (wind, precipitation, pollution, and the like), and contribute to better reflection of incident radiation. The granules moreover are typically colored, naturally or artificially by way of the application of pigments, to meet the aesthetic requirements of the user.
Roofing granules are typically manufactured by using suitable mineral particles coated with coating compositions formed from mixtures of inorganic metal-silicate binders, extender, and metal oxide colorants. Examples of making such granules are disclosed in U.S. Pat. No. 2,981,636. However, such coating processes typically require curing at elevated temperatures, such as above 300 degrees Celsius, or the use of extensive acid wash to render the coating durable for roofing applications, and provide porous coatings. As a result, many colorants or functional additives, which are not resistant to high temperatures or to the corrosive acid-wash process, cannot be used. This limits the palette of colors which can be used in manufacturing roofing granules. Furthermore, the type of coating formed in the conventional process is known to be prone to the so-called “staining” problems as this type of coating tends to absorb oils from the asphalt substrate, apparently reflecting the porosity of the coating.
International Patent Publication WO 2006/106263 A2 discloses biocidal granules consisting of a mineral core coated with at least one porous inorganic coat containing at least one organic compound adapted to limit or to prevent micro-organism growth, in particular algae, as well as a method for sol-gel preparation of such biocidal granules and the use of said biocidal granules in building materials, in particular shingles and façade coatings.
U.S. Pat. No. 5,723,516 discloses inorganic particles coated with a composition including an organometallic polymeric binder and a temperature-sensitive colorant. The binder comprises the reaction product of water with at least one component of a binder precursor. The binder precursor composition can comprise an inert organic polymer and a hydrolyzable liquid organometallic solvent such as TEOS; a water reactive organic polymer dissolved in an inert organic solvent; or a water-reactive organic polymer dissolved in a hydrolyzable liquid organic solvent.
U.S. Pat. No. 6,786,965 discloses an organic pigment dispersion for coloring building material such as concrete including alkali metal silicate particles, organic pigment and a dispersant which is milled to produce a pigment particle size of from 100 to 300 nanometers.
Hence, it would be advantageous to have an inorganic coating system that can be cured in relatively low temperatures. It is further advantageous to have an inorganic coating that provides better staining resistance without the need of additional surface treatment.